Dopamine, norepinephrine, and epinephrine can all be tested. It can be measured in urine to show daily output, or in blood serum to show a snapshot of what is happening at a given moment (like when going from sitting to standing), or in blood platelets to show long term levels over the past three months. I think when it comes to these neurotransmitters, testing is a lot better than guessing based on symptoms. As Sherlock said, there are simply too many things which can cause identical symptoms. In my case low (and another that I know of), low norepinephrine levels seem to cause orthostatic intolerance (and accompanying cognitive problems) of the more NMH variety. My blood platelet norepinephrine levels are quite low, and taking a norepinephrine reuptake inhibitor makes the OI go away for the most part. Norepinephrine is known to regulate blood pressure, but without testing it it's hard to guess if it's responsible or one of a dozen different neurotransmitters or hormones.